This invention relates to thin-film mirrors, and more particularly to a thin-film mirror having a novel support frame for supporting the thin reflecting film of the thin-film mirror, and a thin-film mirror with a novel monocoque construction.
Mirrors are often used in optical systems to fold the optical path, such as the distance between a projector lens and a rear projection screen in a television (TV), in order to compact the size of the optical system and permit a corresponding reduction in cabinet size for the TV. Glass has long been used as the substrate for the reflective material of a mirror, which reflective material is generally a coating of aluminum or silver. The reflective material is usually applied to the front surface of the glass in an optical system to avoid having the projected light rays pass through the thickness of the glass.
As the size of rear projection TV screens increase, the size of reflecting mirrors must also increase proportionally. Relatively large glass mirrors have significant weight. It has thus been customary to make large mirrors for rear projection televisions as thin as possible in order to minimize the weight. However thin glass mirrors are likely to fracture in drop tests that have become a standard requirement for TV manufacturers. Consistent flatness of the mirror reflecting surface is also difficult to obtain with large thin glass mirrors because of the flexibility of the thin glass.
Thin-film mirrors solve many of the problems associated with glass mirrors, such as weight and breakage problems, and are therefore a desirable replacement for glass mirrors in rear projection TV systems.
The term “thin-film mirror” refers to a glassless mirror comprising a thin reflecting film in the form of a plastic sheet with a layer of vacuum deposited reflective metal on one surface, and a supporting frame for supporting the thin reflective film.
Thin-film mirrors are inherently flat because of the flatness of the film substrate. Thus thin-film mirrors have become the mirror of choice by numerous manufacturers of large screen, rear projection TV sets.
Examples of different constructions and constituents of thin-film mirrors are found in U.S. Pat. Nos. 3,180,220; 3,434,181; 3,552,835; 3,608,179; 3,880,500; 5,247,395; and 6,065,843 the disclosures of which are incorporated herein by reference.
In manufacturing known thin-film mirrors it is essential that the film support surface at the corner miter joints of the film support frame be in exactly the same plane to provide a smooth surface for securement of the thin-film mirror to the film support frame. In some instances the film support surface of one joining member at the corner miter joint is inadvertently stepped higher or lower than the film support surface of the adjacent joining member at the corner miter joint. In other instances there may be a slight gap or displacement between adjacent joining members at the corner miter joint.
If a step or gap is present at a corner miter joint where the thin reflecting film is adhered a “crow's foot” wrinkle is likely to form at-the corner portion of the film. Film wrinkling that originates at a corner miter joint will usually radiate from the corner of the thin reflecting film into the optical reflecting area of the thin-film mirror thus causing a distortion of the reflected image on the screen.
I have found that when the mitered corner joint of a mirror frame has an obtuse corner angle, and there are no steps or gaps at the corner miter joint, the thin reflecting film adhered at the corner tensions or shrinks with little or no wrinkling in the mitered corner joint. I have also found that when the mitered corner joint of a mirror frame has an acute corner angle, and there are no steps or gaps at the corner miter joint, the thin reflecting film adhered at the corner tends to develop a “crow's foot” wrinkle in the mitered corner joint.
It is thus desirable to provide a thin-film mirror with no gaps or steps at the corner surfaces of a support frame where the thin film is adhered and a support frame with no acute angles at the corner surfaces where the thin film is adhered. It is also desirable to provide a monocoque construction for a thin-film mirror that eliminates gaps and steps at the corner surfaces where the thin film is adhered.